Cubic boron nitride composite material, method of using it, method of making it and tool comprising it

ABSTRACT

A composite material and a method of using the composite material. The composite material consists of at least 65 volume percent cubic boron nitride (cBN) grains dispersed in a binder matrix, the binder matrix comprising a plurality of microstructures bonded to the cBN grains and a plurality of intermediate regions between the cBN grains; the microstructures comprising nitride or boron compound of a metal; and the intermediate regions including a silicide phase containing the metal chemically bonded with silicon; in which the content of the silicide phase is 2 to 6 weight percent of the composite material, and in which the cBN grains have a mean size of 0.2 to 20 μm.

FIELD OF THE INVENTION

This disclosure relates generally to composite material comprising cubicboron nitride (cBN) dispersed in a binder matrix comprising metalsilicide material; machine tools comprising the composite material;methods of using machine tools comprising the material to machineferrous work-piece bodies in interrupted mode; and methods of making thecomposite material and machine tools comprising it.

BACKGROUND

U.S. Pat. No. 8,419,814 discloses depositing of a nano-sized titaniumnitride, (TiN) and tantalum nitride (TaN) microstructures onto surfacesof cBN grains, by a process involving treating the cBN grains to maketheir surfaces vitreophillic, suspending them in ethanol, introducingTa(OC₂H₅)₅ and Ti(OC₃H₇)₄, and further treating the suspension and cBNgrains such that each cBN grain became coated with an intimate mixtureof titanium and tantalum oxide compounds. The coated cBN grains wereheat treated in suitable atmospheres to convert the oxides intonano-sized TiN and TaN. An aggregation comprising a plurality of cBNgrains thus coated was formed and subjected to pressurisation atultra-high pressure and high temperature, resulting in crack free PCBNmaterial comprising about 84 volume percent cBN within a binder matrixsubstantially consisting of a mixture of TiN and TaN. In machiningtests, the PCBN samples exhibited superior performance, which might havebeen due to the grain size of the binder matrix being close to theHall-Petch nano-grain size.

U.S. Pat. No. 5,288,297 discloses a cBN compact comprising 40 to 90volume percent of cBN crystals bonded by 60 to 10 volume percent of abonding matrix comprised mainly of an intimate mixture of siliconnitride and metallic di-boride, where the metal is chosen from the groupof titanium (Ti), zirconium (Zr) and hafnium (Hf) atoms. The siliconnitride and metallic di-boride each comprise at least 25 volume percentof the bonding matrix, which is strongly and coherently bonded to thecBN crystals. A method for producing the cBN compact by means ofreaction bonding metal silicide compounds with the cBN grains isdisclosed.

There is a need for relatively strong and wear resistant cBN compositematerial having relatively long working life when used to machineferrous work-piece bodies, particularly but not exclusively in heavilyinterrupted machining modes.

SUMMARY

Viewed from a first aspect there can be provided composite materialconsisting of at least 65 volume percent cubic boron nitride (cBN)grains dispersed in a binder matrix, the binder matrix comprising aplurality of microstructures bonded to the cBN grains and a plurality ofintermediate regions between the cBN grains; the microstructurescomprising nitride or boron compound of a metal; and the intermediateregions including a silicide phase containing the metal chemicallybonded with silicon; in which the content of the silicide phase is 2 to6 weight percent of the composite material, and in which the cBN grainshave a mean size of 0.2 to 20 μm. The cBN composite material may also bereferred to as polycrystalline cubic boron nitride (PCBN) material.

Various composite materials are envisaged by this disclosure, of whichthe following are non-limiting, non-exhaustive examples.

In some examples, the content of the silicide phase in the compositematerial may be at most about 5 weight percent of the compositematerial.

In some examples, the silicide phase may be a ceramic or intermetallicphase.

In some examples, the metal may be titanium (Ti). The microstructuresmay comprise titanium nitride (TiN) and or the microstructures comprisetitanium di-boride (TiB₂). In some examples, the silicide may comprisetitanium silicide material having the chemical formula Ti_(x)Si_(y),where x is 0.9 to 1.1 and y is 0.9 to 1.1 (substantially TiSi). In someexamples, the silicide may comprise titanium silicide material havingthe chemical formula Ti_(x)Si_(z), where x is 0.9 to 1.1 and z is 1.9 to2.1 (substantially TiSi₂, titanium di-silicide). The metal boridematerial may comprise titanium di-boride (TiB₂) and relative amounts ofthe titanium di-silicide and the titanium di-boride in the compositematerial may be such that the ratio of the (311) X-ray diffraction (XRD)peak of the titanium silicide to the (101) XRD peak of the titaniumdi-boride is 0.2 to 1.1.

In some examples the metal may be hafnium (Hf), tantalum (Ta) orzirconium (Zr).

In some examples, the silicide phase may be spaced apart from the cBNgrains by the microstructures.

In some examples, the microstructures may be in the form of coatinglayers bonded to surface areas of the cBN grains.

In some examples, the content of the cBN grains may be at least about 80or at least about 90 volume percent of the composite material.

In some examples, the cBN grains have a mean size of 0.1 to 10 microns.In more particular examples, the cBN grains may have mean size of 0.1 to5 microns, or the cBN grains may have mean size of greater than 5 to 20microns. The selection of the mean size and size distribution of the cBNgrains, such as whether it may have a one or more than one mode, maydepend on the type of application in which the composite material isintended to be used.

In some examples, the mean size and or the size distribution of the cBNgrains may be selected according to the cBN content, such that the meanvolume of the binder matrix between the cBN grains will be neither toolarge nor too small. In some examples, the cBN grains comprised incomposite material having a relatively high content of cBN grains (atleast about 80 or 90 volume percent) may be relatively larger than incomposite material having a relatively lower content of cBN grains. Insome examples, the mean size of the cBN grains in composite materialcomprising at least 80 or at least 90 volume percent cBN grains may begreater than 5 microns or greater than about 10 microns. In otherexamples, the mean size of the cBN grains in composite materialcomprising less than 90 or less than 80 volume percent cBN grains, maybe greater than 0.1 microns or greater than about 5 microns, and lessthan about 10 microns. While wishing not to be bound by a particulartheory, the mean size and or size distribution of the cBN grains may bebalanced against the content of the cBN in the composite material inorder to achieve an overall amount of residual silicide according tothis disclosure. This may help control the silicide content of thecomposite material, achieving silicide content that is not too high(potentially as a result of the regions between the cBN grains being toolarge) and not too low (potentially as a result of the regions betweenthe cBN grains being too small).

In some examples, the area distribution of the cBN grains as viewed on asurface of the composite material may have at least two modes.

In some examples, the binder matrix may comprise silicon nitride(Si₃N₄).

In some examples, the content of the silicide phase may be 20 to 60weight percent of the binder matrix.

Viewed from a second aspect, there is provided a method of using exampledisclosed composite material, the method including providing a machinetool comprising a cutting edge comprising the composite material; usingthe machine tool to machine a work-piece comprising ferrous material ininterrupted mode, in which the work-piece is configured such that thecontinuous engagement length is 30 to 50 percent of a distance traversedbetween the cutting edge and the surface of the work-piece during themachining operation.

In some examples, the work-piece may be configured such that at least apart of the work-piece presents an engagement angle to the cutting tool,the engagement angle being 30 to 90 degrees.

In some examples, the work-piece may comprise material having hardnessof at least 50, at least 52, at least 60 or at least 62 on the Rockwell‘C’ hardness scale (HRC).

In some examples, the work-piece may comprise steel, cast iron orsuper-alloy material. For example, the work-piece may comprise steel andor grey cast iron material.

In some examples, the machine tool may comprise the composite materialjoined to a support body.

In some examples, the machine tool comprises an indexable insert. Forexample, the machine tool may be configured for use in a turning ormilling operation, and the method may include using the machine tool ina turning or milling operation. In some examples, the work-piece may besuitable for the manufacture of a brake disc, and the method may includemachining the work-piece to make a brake disc.

Viewed from a third aspect, there is provided a method of making anarticle comprising example disclosed composite material, the methodincluding combining a silicide phase precursor with a plurality of cBNgrains to provide a raw material combination, the silicide phaseprecursor selected such that the metal will be capable of reacting withthe cBN grains to form a nitride or boride reaction product; in whichthe content of the cBN grains in the raw material combination is suchthat the content of the cBN grains in the composite material will be atleast 65 volume percent of the composite material; subjecting the rawmaterial combination to a pressure at which cBN is a thermodynamicallystable phase, and a temperature sufficiently high and for a sufficientperiod of time for some of the metal contained in the silicide phaseprecursor to react with the cBN grains to form the plurality ofmicrostructures reaction-bonded to the cBN grains; and for remainingmetal silicide phase to be 2 to 6 weight percent of the compositematerial or 20 to 60 percent of the binder matrix, being the balance ofmaterial other than the cBN comprised in the compound material.

In some examples the remaining metal silicide phase is 2 to 5 weightpercent of the composite material or 20 to 50 percent of the bindermatrix, being the balance of material other than the cBN comprised inthe compound material.

In some examples, the silicide phase precursor may be in powder form,and the mean grain size of the grains of the silicide phase powder maybe 0.1 to 5 microns.

In some examples, the metal comprised in the silicide phase precursormay be titanium (Ti). In some examples, the silicide may comprise atitanium silicide material having the chemical formula Ti_(x)Si_(y),where x is 0.9 to 1.1 and y is 0.9 to 1.1 (substantially TiSi). In someexamples, the silicide may comprise a titanium silicide material havingthe chemical formula Ti_(x)Si_(z), where x is 0.9 to 1.1 and z is 1.9 to2.1 (substantially TiSi₂, titanium di-silicide).

In some examples, the silicide phase may include hafnium (Hf), tantalum(Ta) or zirconium (Zr).

In some examples, the method may include producing the silicide phaseprecursor, by combining the metal and Si in elemental form such that themetal and the Si will be capable of reacting with each, forming apre-reaction combination; treating the pre-reaction combination suchthat the metal reacts with the Si to form reacted material comprisingthe silicide phase; and comminuting the reacted material to provide aplurality of the grains of the silicide phase. The method may includecomminuting the reacted material by means of attrition milling.

In some examples, the temperature at which the composite material issintered at the ultra-high pressure may affect the overall content ofthe residual silicide phase in the composite material. In particular,relatively lower sintering temperatures (all else being substantiallyequal) may result in a higher content of the residual silicide phase.

Viewed from a fourth aspect, there can be provided a machine toolsuitable for machining ferrous work-piece in interrupted mode,comprising example disclosed composite material, in which a cuttingedge, a rake face and a flank face comprise the composite material. Themachine tool may be for turning or milling the work-piece.

Non-limiting examples of composite material and machine tools will bedescribed with reference to the accompanying drawings, of which

DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B show scanning electron micrograph (SEM) images ofcomposite material according to Example 2 described below, at twodifferent magnifications as indicated by the scale bar showing 1 micron;

FIG. 2A and FIG. 2B show two SEM images of example composite material,including number references corresponding to points at which energydispersive spectroscopy (EDS) spectra were obtained;

FIG. 3 shows an X-ray diffraction graph of example composite material,in which the main peaks have been identified;

FIG. 4 shows a bar chart comparing the working lives of cutter elementsin a machine test referred to as ‘H30 heavy interrupted hard partturning’ (six of the cutter elements comprised example compositematerial and one cutter element comprised a reference compositematerial);

FIG. 5 shows a bar chart comparing the wear scars formed into cutterelements in a machine test referred to as ‘K30 heavy interrupted greycast iron brake disc turning’ (six of the cutter elements comprised thesame example composite material and reference material as in FIG. 4);and

FIG. 6 shows a photograph of a perspective view of the test work-pieceused for carrying out H30 machining tests on example and referencecomposite materials.

DETAILED DESCRIPTION

With reference to FIG. 1A, FIG. 1B, FIG. 2A and FIG. 2B, examplecomposite material may comprise a plurality of cBN grains 10 comprisingabout 90 volume percent of the composite material, dispersed in a bindermatrix, the binder matrix comprising a plurality of microstructures 12reaction-bonded to the cBN grains 10 and a plurality of intermediateregions 14 within interstices between the cBN grains 10. Themicrostructures 12 may comprise titanium nitride (TiN) and titaniumdi-boride (TiB₂); in particular the microstructures 12 may contain moreTiB₂ than TiN and potentially a minor amount of Si₃N₄ grains, whichappears to be present in in the binder matrix in a trace amount of notmore than about 1 to 2 volume percent of the composite material. Someparts of the intermediate regions 14 appear to comprise a titaniumsilicide phase, particularly TiSi₂ and probably TiSi, and other partsappear to comprise TiB₂. Other regions 16 appear to contain siliconnitride (Si₃N₄). The content of the titanium silicide phase will be 2 to6 weight percent or 2 to 5 weight percent of the composite material.

With reference to FIG. 2A and FIG. 2B, energy dispersive spectroscopy(EDS) analysis was carried out in situ to give an indication of theprinciple elements at various points on area sections of surfaces ofexample composite material and the results are summarised in Table 1.Based on the relative molar amounts of the elements detected, potentialnon-limiting and mon-exhaustive compounds or phases are mentioned forsome of the points.

TABLE 1 Point Potential Ti Si N B O ref. phases wt. % mol. % wt. % mol.% wt. % mol. % wt. % mol. % wt. % mol. % a TiSi₂ 45 33 55 67 b TiSi₂ 4833 49 57 3 10 c Ti-rich, Si₃N₄ 25 12 47 39 29 49 d B-rich, cBN 8 2 8 439 38 44 55 2 1 e TiSi₂ 43 30 58 70 f TiSi₂/Si₃N₄ 30 15 41 35 29 49 gTiSi/Si₃N₄? 35 17 28 23 30 50 7 9 h TiB₂, Si₃N₄ 32 14 29 22 21 32 13 255 7 i cBN 56 50 44 50 j cBN 57 51 42 49 k cBN 57 51 43 49 l TiSi₂ 53 3948 61 m TiN, Si₃N₄ 17 7 40 30 43 63 n TiB₂?, cBN 10 3 17 9 32 34 40 54

An example method of manufacturing example cBN composite material willbe described.

A mixture or alloy comprising silicon (Si) and titanium (Ti) can be madeby blending together Ti and Si powders in a suitable ratio, such as2Ti+3Si, and treating blended powders in vacuum at an elevatedtemperature, for example. The reaction product may include one or moretitanium silicide alloy or intermetallic phase of one or more of thecompounds TiSi, TiSi₂ or Ti₅Si₃, which may be produced in combinationwith each other. The Si and Ti powders can be intimately blended andsealed in an evacuated silica tube or metal-encased vacuum furnace (forexample, a molybdenum alloy interior wall vacuum furnace) for the heattreatment at a temperature of about 1,000 to about 1,200 degreesCelsius. For example, the blended powders may be heat treated undervacuum at a temperature of about 1,100 degrees Celsius for about 15 to30 minutes. In an experiment in which the blended Ti and Si powders wereblended together and subjected to vacuum heat treatment at a temperatureof 1,000 degrees Celsius for 2 hours, X-ray diffraction (XRD) analysisrevealed that some residual unreacted Si was evident in the reactionproduct material, while no evidence of unreacted Si was evident when theheat treatment was carried out at 1,100 degrees Celsius for 2 hours.

The bulk composition of the reaction product material may beapproximately equivalent to Si₃Ti₂, in the form of coarse granules,which may be comminuted by means of attrition milling for up to about 4hours, for example, to provide a finely dispersed blended powder, theparticles of which having a mean size of at most about 10 microns or atmost about 3 microns. Using attrition milling to comminute the titaniumor other silicide granules is a relatively aggressive, high power methodand will likely be capable of reducing the size of the granules to veryfine powder having a mean grain size of about 1.5 to 2.5 microns insize. Intermetallic TiSi₂ and other titanium silicide phases are morebrittle than the precursor Ti and Si in elemental form, which may makeit easier to control the comminution of the silicide phases to producevery finely divided grains by means of attrition milling than it wouldlikely be for other sources of the precursor elements. In addition, thesilicide phases tend to be substantially more resistant to oxidationthan elemental Ti and Si; oxidation of Ti and or Si would likely reduceeffective sintering or bonding with the cBN for form the cBN compositematerial. The reaction of the Ti with the Si may be carried out invacuum in order to promote the removal of oxygen that may be present.

While wishing not to be bound by a particular theory, relatively finetitanium or other silicide powder of about 1 to about 5 microns mayresult in a more uniform blend of the grains and consequently morehomogeneous microstructure and or superior properties of the sinteredcBN composite material. In particular, very finely divided silicidephase grains would have a relatively high specific surface area forreacting with the cBN grains, potentially resulting in more effectivereaction sintering of the cBN composite material. Titanium silicidepowder (or other kinds of silicide powder such as hafnium silicide orzirconium silicide powder) having grain size of less than about 0.1micron may increase the risk of introducing too much surface oxygen intothe raw material powder blend, since the specific surface area will berelatively large and oxide compounds and other impurities may likely beattached to the powder surfaces. The presence of too much oxygen in theraw material powder will likely be deleterious to certain of theproperties and behaviour in use of the sintered composite material.

The fine titanium silicide powder can then be blended with a pluralityof cBN grains, which may have a mean size of 0.1 to about 5 microns, andthe size distribution of which may exhibit one, two or more modes (whichmay also be referred to as peaks). For example, sonication means may beused for blending the cBN grains with the titanium silicide powder. Insome example methods, the cBN grains and the silicide powder may beblended by means of shear mixing, in which the powders may be mixed inhexane or other suitable fluid medium and then dried and sieved toselect grains of a suitable size, such as about 220 microns. In someexample methods, the cBN grains and silicide powders may be mixed bymeans of a shaker-mixer (such as a Turbula™ blender) with the aid ofsteel balls, after which the balls will be removed and the blendedpowder provided. The use of sonication mixing may result in superiorhomogeneity in the microstructure of the sintered cBN compositematerial, which may contain fewer and smaller agglomerations ofmaterial, or substantially free of material agglomerations evident inthe binder matrix.

The relative amounts of the cBN grains and the titanium silicide powderwill be selected such that the desired weight or volume content of cBNwill be present in the sintered cBN composite material, which will be atleast 65 volume percent of the composite material, and may be at least70, at least 80 or at least 90 weight percent. Loss of a minor amount ofthe cBN due to reaction of a small amount of the cBN with the Ti and orthe Si to form TiN and or TiB₂ and or Si₃N₄ can be taken into accountwhen blending the cBN grains with the titanium silicide powder. Theamount of cBN likely to be lost in this way will likely depend on thespecific surface area of the cBN grains, which will likely depend on theshape and size distribution, and the quantity of the cBN grains (inother words, a little more cBN than may at first appear necessary can beadded to compensate for this potential effect).

The powder or granules comprising the mixed silicide powder and cBNgrains may then be formed and compacted to provide a pre-sinter bodysuch as a disc. The pre-sinter body need not be placed against acemented carbide substrate and may be encapsulated within a refractorymetal jacket and out-gassed at about 750 degrees Celsius in vacuum forabout 30 minutes. After the outgassing step, the encapsulated disc maybe sealed within an additional refractory metal jacked to provide adouble-jacketed pre-sinter body. The encapsulated pre-sinter body willbe subjected to a high pressure at which cBN is thermodynamicallystable, such as at least about 3 gigapascals (GPa), at least about 5.5GPa or at least about 6.5 GPa, and a high temperature at which titaniumor other metal in the metal silicide can react with the cBN to formSi₃N₄, TiN and or TiB₂. In general, higher sinter pressures in the range3 to 8 GPa may likely result in denser sintered composite compacts andexhibit certain superior properties and behaviour in use. In someexamples, the pressure may be about 6.5 to about 7.0 GPa and thetemperature may be at least about 1,300 to about 1,450 degrees Celsius(in general, higher temperatures may be used when higher pressures areused; so a temperature of about 1,450 degrees Celsius may be used for apressure of about 6.8 GPa.

The temperature used for sintering the raw material powders will have aneffect on the relative amount of residual silicide not having reactedwith the cBN; in general and all else being equal, the lower the sintertemperature, the higher may be the content of residual metal silicide inthe sintered composite material.

Although the example method described above mentioned principallytitanium silicide, the method for making raw material comprising othermetal silicide phases such as hafnium silicide or zirconium silicidewill be substantially similar.

Certain example cBN compacts appear to be particularly effective for usein heavily interrupted machining operations such as turning or millingof hardened steel having hardness of at least 50 or at least 52 HRC(hardness on the Rockwell C scale) and or cast iron, such as grey castiron.

While wishing not to be bound by a particular theory, chemical reactionof the cBN grains with elements from the silicide phase powder mayresult in strong bonding between the cBN grains and the binder matrix.For example, metal boride and or metal nitride reaction productmicrostructures may be strongly bonded to the cBN surfaces. For suchreaction bonding to occur, the silicide phase used as raw material forsintering the cBN composite material would need to contain a metal thatis capable of reacting with a source of boron to form a metal boridecompound and or with a source of nitrogen to form a metal nitridecompound. Potentially, the nitride and or boride reaction products ofsuch metals and the cBN may be in the form of layer- or coating-likemicrostructures bonded to the cBN grains. The presence of a small amountof relatively brittle material such as the remaining silicide phase,which did not react with the cBN grains, may enhance the impactresistance and strength of the cBN composite material. The silicidecompounds such as titanium silicide may be present as intermetallicphases, which may be relatively brittle and potentially more brittle(less strong or tough) than titanium nitride and or titanium boride, andor other materials present in the binder matrix. Potentially, thepresence of a small amount of relatively brittle material may have theeffect of improving the impact resistance and or strength, particularlythe impact strength of the cBN composite material. Impact strength willlikely be an important property of material used for the interruptedmachining of work-pieces. While wishing not to be bound by a particulartheory, this may occur by the silicide phase consuming impact energy bythe proliferation of crack within the silicide phase, in effect‘shattering’ the silicide phase. For example, when a crack propagatingthrough the cBN composite material arrives at a region or grain of thesilicide phase, a substantial amount of its energy may be consumed insuch ‘shattering’, thus attenuating or preventing its furtherpropagation. If too much of the silicide phase were present, then theoverall impact resistance of the composite material will likely decreaseas the ‘shattering’ effect will likely have increasingly longer rangeand occur in too great a volume of the composite material. In additionor alternatively, too high a content of the silicide phase maypotentially have a deleterious effect on certain other properties of thecomposite material, such as chemical or other wear resistance. If toolittle of the phase were present, potential crack attenuation orinhibition effects may become much less significant or negligible.

While wishing not to be bound by a particular theory, example disclosedcBN composite materials appear to combine an aspect of strong reactionbonding between the cBN grains and the binder matrix, with an amount ofremaining residual silicide material that may be effective for enhancingcertain mechanical properties of the composite material, such as impactresistance, which may be particularly helpful for interrupted machining.

While wishing not to be bound by a particular theory, an example methodused to make the composite material, involving attrition milling themetal silicide material prior to blending it with the cBN grains andthus providing finely divided metal silicide raw material powder, mayhave an aspect of enhancing the homogeneity of the binder microstructureor altering the binder microstructure in some other way.

Non-limiting and non-exhaustive examples will now be described in moredetail.

Examples 1 to 6

Six pairs of test machine tools comprising example cBN compositematerial cut from the same sintered disc were made in order to carry outtwo kinds of interrupted machining tests as described below. A pair ofreference tool comprising reference cBN composite material were alsomade and tested. All of the example cutting tools had the sameconfiguration and comprised the nominally the same cBN compositematerial, which comprised 90 volume percent cBN grains, TiN, TiB₂, Si₃N₄and titanium silicide intermetallic phases TiSi and TiSi₂.

The reference material was AMB90™ PCBN, an Element Six product that isused for heavily interrupted hard turning. The reference material wasmade by sintering cBN grains blended with aluminium (Al) powder having amean grain size of about 6 microns, the mass content of the cBN beingabout 90 percent of the blended powders and the balance consisting ofthe Al powder. The cBN grains had a mean size in the range of 3 to 8microns. The blended powders was compacted to form a pre-sinter disc andsubjected to a sinter pressure of about 5.5 GPa and a sinter temperatureof about 1,250 degrees Celsius for a period of about 30 minutes.

The example cBN composite material was made as follows. Ti and Si powderwas blended together in the molar ratio of 3Ti and 2Si and subjected toheat treatment of about 1,100 degrees Celsius in a vacuum furnaceevacuated to 10⁻³ to 10⁻⁶ millibars (mbar). The reaction productincluded at least TiSi and TiSi₂ in the form of relatively large pieces,which were crushed and sieved to about 212 microns, and then comminutedby means of attrition milling in hexane for 4 hours to provide wellblended powder having mean grain size of about 1.5 to 2.5 microns. Theattrition milled powder was recovered, dried of in a rotary evaporatorand then additionally dried at 60 degrees Celsius in an oven overnight.

The fine titanium silicide powder was blended with 90 weight percent ofcBN grains having a mean size of 2 to 20 microns, as shown in Table 4for the various Examples. Various methods were used to blend the powdersfor Examples 1 to 6. The cBN and Ti—Si reaction product powder wasblended by means of sonication in hexane in Examples 1, 2 and 6; bymeans of a (Turbula™) shaker-mixer and steel balls in Example 4; aplanetary ball mixer for Example 5.

The sonication mixing in Examples 1, 2 and 6 involved adding hexane toTi—Si reaction product powder and introducing an ultrasonic prove intothe resulting suspension, and applying a sonication amplitude of 25%(using a Branson™ apparatus having 2,000 bdc, maximum frequency of 20kHz, maximum power of 2.2 kW, and full-wave 50 mm diameter titaniumhorn) for 5 minutes. An amount of cBN powder was then introduced intothe suspension, such that the cBN content was 90 weight percent of thecombined Ti—Si powder ad cBN mixture, and the combined suspension wassonicated for 10 minutes at the same amplitude. The suspension was driedin a rotary evaporator and then in an oven at 60 degrees Celsius for atleast 5 hours. The mixed powder was allowed to cool to about 25 degreesCelsius in nitrogen atmosphere and sieved to less than about 212microns.

The shaker-mixing of Example 4 involved combining 90 weight percent cBNpowder with 10 weight percent Ti—Si reaction product powder andintroducing 8 WC balls, each of 8 mm diameter. The powders wereshaker-mixed fort 1 hour, following which the WC balls were removed.

The planetary ball mixing of Example 5 involved combining the 90 weightpercent cBN grains with 10 weight percent Ti—Si reaction product powderand introducing WC balls having diameter of 3 mm, such that the massratio of the combined powder to the balls was 1:2.5. Hexane was added tothe mixed powders, such that the volume of powders to that of the hexanewas about 2:1. The suspension was subjected to planetary ball millingfor 30 minutes at 90 revolutions per minute (rpm). The balls wereremoved and the slurry or suspension was dried in a rotary evaporator,followed by drying in an oven at 60 degrees Celsius for at least 5hours. The mixed powder was allowed to cool to about 25 degrees Celsiusin nitrogen atmosphere and sieved to less than about 212 microns.

The blended raw material powders comprising the cBN and titaniumsilicide grains was compacted to form a plurality of discs. Thecompacted discs were encapsulated within a reaction capsule for anultra-high pressure furnace (which may also be referred to as anultra-high pressure press), and subjected to an ultra-high pressure ofabout 6.8 GPa and a temperature of about 1,450 degrees Celsius for aperiod of about 10 minutes to provide a sintered disc consisting of thecBN composite material (a slightly lower temperature was used to sinterExample 3). The disc was cut up to form cutter element precursor bodies,which were then further processed by means of diamond grinding toprovide the six cutter elements for the six example cutting tools.

With reference to FIG. 3, the ratio of the height of the titaniumsilicide (TiSi₂) (311) X-ray diffraction XRD peak near about 2Θ of 45.77degrees to that of the titanium di-boride (TiB₂) (101) peak 2Θ of 52.1degrees on the XRD spectrum were measured for Examples 2 and 6. Theseratios were measured to be about 0.37, which appears to indicate thatthe composite material had a relatively low content of TiSi₂ and perhapsother titanium silicide material (in other Examples 7 and 9 describedbelow, in which the cBN content was 65 volume percent, this ratio wasabout 1.0). It is estimated that this may indicate that the content oftitanium silicide material may be about 2 to 3 weight percent of the cBNcomposite material.

Example turning tools were made comprising the Example cBN compositematerial. The sintered cBN composite discs were cut into pieces havingdimension 10×10 mm, each having thickness of 3.2 mm. The tool cuttingedge was prepared by forming a 25 degree chamfer angle and 20 micronedge hone, and a −6 degree rake angle. The tools were tested in a K30grey cast iron cast iron test (similar to brake disc machining) and aso-called H30 ‘O1 clock test’.

One of each pair of Example tools was tested in a turning test, in whichthe cutter insert was used to machine (turn) a body consisting ofhardened steel, under conditions selected to have similarities with H25or H30 hard turning. Each test was terminated when the edge of thecutter, defined by example cBN composite material, had become fracturedto the extent that the size of the fracture scar (measured parallel tothe cutting velocity vector) is greater than the mean size of the flankwear scar, or length of the flank wear scar reaches at least 0.3millimetres. The occurrence of either of these occurrences was the endof life criterion, which may be evident in a relatively sudden change inthe measured cutting force. Catastrophic edge fracture may occur beforea flank wear land dimension of 0.3 millimetre (mm) has formed. Theperformance of the cBN composite material can be reported in terms ofthe number of passes required for the end of life criterion to beachieved; the greater the insert life, the better the performance of thecBN composite material in the test. This result is expected to providean indication of the potential working life of the cBN compositematerial in certain industrial machining applications involving theinterrupted cutting of steel bodies.

The test work-piece is shown in FIG. 6 comprised a series of bars 30projecting axially from a circumference of a circular plate (hence thename ‘clock test’). Each of the bars presented an engagement angle of 90degrees to the tools and had substantially the same hardness throughoutits volume (which may also be referred to as “through hardened”), theRockwell C hardness being in the range of about 60 to 60 HRC, being ahardened steel material according to the AISI 4340 specification. Thetest is believed to provide a reasonably good indication of thepotential performance of the PCBN material in machining case hardenedsteels (in particular but not exclusively) in many applications inpractice. The test work-piece and the cutting conditions were configuredto subject the tool to a particular ratio of continuous and interruptedcutting conditions, this ratio being substantially constant for eachcutting cycle (which may be referred to as a “pass”). In particular,this ratio was kept substantially constant throughout the test byadopting a face-turning approach with constant surface speed control. Aseries of arcuate spaces between the bars 30 were provided parallel tothe longitudinal axis of rotation of the test work-piece axis such thatthe diameter and pitch spacing of the holes is expected to presentturning conditions likely to be representative of certain common hardturning operations in industry.

The other of each pair of Example tools was used in a K30 interruptedmachining test in which the work-piece consisted of grey cast iron andthe response variable was the size of the wear scar after a certaindistance of cutting (material removed from the work-piece). Furtherparticulars of the H30 and K30 tests are presented below. The testparameters used in the ‘brake disc’ K30 and H30 ‘clock test’ using O1tool steel is provided in Table 2. Information about the grade of thegrey cast iron material used in the K30 test is shown in Table 3. Thegrade used provides relatively good wear resistance, strength and heattreatment response compared to other grades GG20 and GG25, and hasreasonable machinability and excellent surface finish. The Brinellhardness of the grade of grey cast iron used was 190 to 260. In general,PCBN tools are used for machining grey cast iron at relatively highcutting speeds, as shown in Table 2.

TABLE 2 K30 Break disc test H30 Clock test Cutting Cutting speed, 1,200180 conditions m/min. Feed, mm/rev. 0.3 0.3 Depth of cut, 0.4 0.2 mmWork-piece Brake disc 300 mm O1 tool steel (about 62 HRC) Insertgeometry SNMN0904*08S02025 Failure Wear after 6 discs Catastrophicfailure Tool holder 75° approach, −6 rake

TABLE 3 DIN EN Grade GG30 EN-GJL-300 Comparable with Meehanite GB300Standard DIN 1691 EN 1561 Alloy number 0.6030 EN-JL 1050

Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) analysisconfirmed the presence of a substantial amount of titanium di-boride(TiB₂) and silicon nitride (Si₃N₄) phases, which likely arose due toreaction bonding between the cBN grains and the sources of titanium (Ti)and silicon (Si). The example tools exhibited improvements of up toabout 100 percent in terms of tool life in H30 applications and 30percent in terms of wear resistance in K30 applications.

The results of the H30 and K30 cutting tests of each of the six pairs ofExample tools is summarised in Table 4, together with the correspondingresults achieved using the reference material. The machining testresults are also shown graphically in FIG. 4 and FIG. 5, and indicatethat the Example cBN composite material performs substantially betterthan the Reference sample in both tests. In the H30 test, the meannumber of passes exhibited by the Example cutters was about 76 with astandard deviation of about 6, whereas the number of passes for theReference cutter was about 38. In the K30 test, the mean wear scar sizethat formed in the Example cutters was about 68 microns with a standarddeviation of about 4 microns, whereas the wear scar in the Referencecutter was about 85 microns.

TABLE 4 Volumetric D[4,3] mean cBN grain size, μm Relative wt.% of cBNin the size cBN content, wt % XRD peaks  $\frac{{TiSi}\; 2\mspace{14mu} (311)}{{TiB}\; 2\mspace{14mu} (101)}$H30 test: no. of passes to failure K30 test: wear scar size, μm Example1 20 75% 90 68 69 6-7 25% Example 2 20 90% 90 0.37 72 70  3 10% Example3 20 75% 90 72 68 6-7 25% Example 4 12-15 100%  90 82 70 Example 5 2075% 90 83 70 6-7 25% Example 6  7.5-10.5 100%  90 0.37 80 60 Reference90 38 85

These results appear to provide strong evidence that cBN compositematerial comprising 90 volume percent cBN grains reaction bonded to abinder matrix including a small amount of one or more titanium silicideintermetallic phases has relatively high strength and toughness, andwill likely perform well in machining operations involving interruptedmachining of hardened steel and grey cast iron.

Examples 7 to 10

Titanium (Ti) and silicon (Si) powder having mean grain size of about 30to 50 microns and 99.5 and 99 percent purity, respectively, were mixedin the molar ratio 2Ti+3Si (a weight ratio of 53 percent Ti and 47percent Si) by means of a shaker-mixer for 1 hour with the aid of steelballs. The mixed powders were subjected to heat treatment in a vacuum at1,100 degrees Celsius for 2 hours and the Ti—Si reaction productmaterial was subjected to attrition milling for 4 hours. X-raydiffraction (XRD) analysis of the Ti—Si reaction product materialindicated that the mean content of oxygen was about 8 weight percent(plus or minus about 5 weight percent).

In the various Examples, the attrition milled silicide reaction productmaterial was blended with cBN grains having mean size as shown in Table5 by means of a shaker-mixer (a Turbula™ mixer) and steel balls forabout 1 hour and the mixed powder compacted into discs each having amass of 50 grams (g). The cBN content in the raw material was 90 weightpercent and the balance was the TiSi reaction product material. Eachdisc was encapsulated within a refractory metal jacket, outgassed at 750degrees Celsius in vacuum for 30 minutes, sealed within an additionalrefractory metal jacket and subjected to pressurisation at about 5.5gigapascals (GPa) at a temperature of about 1,250 degrees Celsius toprovide the Examples 7 to 10.

Scanning electron microscopy (SEM) analysis revealed no evidence ofsubstantial agglomeration in any of these Example composite materials,suggesting that the finely-divided Ti—Si reaction product material hadbeen very effectively and homogeneously blended with the cBN grains.Clear evidence of reaction sintering of the cBN grains with the bindermatrix was evident.

TABLE 5 Volumetric D[4,3] mean cBN grain Flank wear, size, μm Vb, μmExample 7 12-15  67 Example 8 7.5-10.5 70 Example 9 12-15  71 Example 107.5-10.5 73

Example machine tools were made comprising each of the example cBNcomposite materials and subjected to a test in a K30 type application,which will likely provide an indication of the performance of thecomposite material in the machining of brake discs. The responsevariable of the test was the size of the wear scars that formed at thecutting edge of the machine tool after a certain number of cuttingpasses (the so-called ‘Vb’ length), which is shown in Table 5 for eachof the Example tools. In terms of this measure of performance, all ofExamples 7 to 10 exhibited superior performance to the reference AMB90™PCBN material.

Examples 11 to 15

Example cBN composite material comprising 65, 80 and 90 volume percentwere made using the method substantially as explained in relation toExample 1 to 6, except that temperature used for sintering Example 15was estimated to be about 100 to 200 degrees Celsius lower than used forthe other Examples.

The materials were subjected to XRD analysis. The ratio of the height ofthe titanium silicide (TiSi₂) (311) X-ray diffraction XRD peak at about2Θ of 45.77 degrees (the most intense peak for TiSi₂) to that of thetitanium di-boride (TiB₂) (101) most intense peak at 2Θ of about 52.1degrees on the XRD spectrum were measured and the results are presentedin Table 6.

TABLE 6 Volumetric D[4,3] mean cBN grain size, μm cBN content, vol. %XRD peaks  $\frac{{TiSi}\; 2\mspace{14mu} (311)}{{TiB}\; 2\mspace{14mu} (101)}$Example 11 5 65 1.00 Example 12 5 80 0.43 Example 13 5 65 1.06 Example14 5 80 0.44 Example 15 10 90 0.93

The amount of residual silicide remaining in the composite materialappears to be affected by at least the mean size and potentially thesize distribution of the cBN grains, the content of the cBN grains inthe composite material and the temperature at which the compositematerial is sintered. This latter point is indicated by the fact thatthe TiSi₂/TiB₂ ratio for Example 15 is higher than may be expected frominspection of the other data, since the trend is for the ratio to behigher for relatively low cBN content and lower for relatively high cBNcontent. While wishing not to be bound by a particular theory, itappears that the reduction of the sinter temperature for Example 15resulted in a higher ratio (that is, a relatively higher content ofsilicide phase).

Certain terms and concepts as used herein are briefly explained below.

As used herein, a machine tool is a powered mechanical device that maybe used to manufacture components by the selective removal of materialfrom a work-piece, a process that may be referred to as machining. Abody to be machined in the manufacture of an article may be referred toas a work-piece material and, in general, may comprise metal, alloys,composite materials, wood, polymers, including carbon fibre-reinforcedpolymers. A cutting tool may have a rake face, being a surface orsurfaces over which chips from the work-piece, the rake face directingthe flow of newly formed chips. ‘Chips’ are the pieces of a body removedfrom the work surface of the body by a machine tool in use. The flank ofa cutter insert is the surface that passes over the machined surfaceproduced on the body by the cutter insert. The flank may provide aclearance from the body and may comprise more than one flank face. Acutting edge is the edge of a rake face intended to perform cutting of abody.

As used herein, ‘roughing’ refers to an aggressive form of machining inwhich work-piece material is removed at a relatively high rate by usinga large depth of cut and feed rate. This is distinguished from“finishing”, where the objective being to produce a high tolerancefinish, and so the depth of cut and feed rates are lower. In roughingoperations, the load on the cutting edge of a tool is far greater thanin finishing operations and so the cutting edge needs to be muchstronger in a roughing operation, especially when the rake angle ispositive. This makes hard or super-hard, but relatively brittlematerials generally unsuitable for roughing certain difficult-to-machinework-piece materials, such as titanium alloys. For example PCD, PCBN oradvanced ceramics are not typically used for the rough machining ofdifficult-to-machine materials, despite the high abrasion resistance ofthese materials.

In rough machining operations, the feed rate and depth of cut arerelatively high and the load on the cutting edge of the tool is high,often in the range of about 5 to 10 kN (kilonewtons). Rough machining isfrequently undertaken on work-pieces which include an “interrupt”aspect, which may be intentional or unintentional. For example, aninterrupt may be in the form of a “V” groove or porosity from gasesevolved during casting, slag or sand particles. In rough machining,dimensional tolerance is not as critical as in finishing operations andflank wear values up to and in excess of 1 mm may be permitted.Consequently, it is likely that chip resistance rather than wear is thedominant failure mode in rough machining.

Cutting tools comprising cBN composite material such as PCBN materialmay be used to machine three broad groups of ferrous materials, namelyhardened steel (“hard turning”), sintered powder metals comprising hardgrains in a relatively softer matrix, and grey and hard cast irons.Example types of hardened steels may have Rockwell ‘C’ hardness of atleast 50 HRC or at least 52 HRC.

A machining operation may involve a cutting tool remaining engaged witha work-piece article (in other words, the body being machined)throughout the entire operation or it may repeatedly engage anddisengage the work-piece during the operation. For example, thework-piece may have a relatively complex shape, potentially havingrecesses and protrusions, and or the machining operation may includemilling, in which a rotating cutting tool will repeatedly engage thework-piece only through an arc of the rotation. A mode of machining inwhich a cutting tool will engage the work-piece and remove material fromit through part of the operation and be disengaged from it for the restof the operation may be referred to as ‘interrupted machining’ or‘interrupted cutting’. Various factors associated with the configurationof the work-piece will likely influence the machining process and theselection of a suitable cutting tool and material for the cutting edge.In particular, such factors may include the ‘engagement angle’, thepercentage of interrupted cut and the ‘continuous engagement length’.Factors associated with the machining operation may include the ‘feedrate’, the ‘rake angle’, the ‘cutting speed’ and the ‘depth of cut’, toname a few.

The engagement angle and the continuous edge length are parameters thathelp describe the shape of a lateral cross section through an elongatework-piece to be subjected to turning, in which the work-piece will berotated rapidly about a central longitudinal axis connecting oppositeends of the work-piece and a cutting tool is positioned so as to engageand cut material adjacent the surface of the work-piece. The cuttingtool may also be moving radially inwards as the outer radial dimensionof the work-piece will be reducing as a result of the cutting operation,and may me moving along the side of the work-piece, in a directionaligned with the longitudinal axis of the latter.

A test work-piece body to be machined may have the general shape of acentral round core from which a plurality of spoke formations protruderadially outwards (the work-piece configuration shown and described isused for simplicity and the concepts being described will apply towork-piece configurations in general, including irregular or asymmetricwork-pieces). The engagement angle φ is that between the cutting edgeand the side surface of the approaching spoke, about to be engaged andcut by the cutting tool. For example, where work-piece is configuredsuch that sides of the spoke formations lie on radial planes convergingon the central longitudinal axis, the engagement angle will be 90degrees; where the sides of the spoke formations slope inwards otherthan aligned with radial planes, the engagement angle will be less than90 degrees. An engagement angle of 90 degrees will present the cuttingtool with a particularly harsh impact in the interrupted cuttingoperation. At the opposite extreme, an engagement angle of 0 degreeswill in effect correspond to no interruption at all and the tool will ineffect be machining the work-piece in a continuous mode, at least forthat portion of the revolution of the work-piece. In fully continuousmachining, the cutting edge will remain engaged with the work-piecethroughout the entire revolution of the latter (turning is used as anexample to explain the concept of engagement angle, which may also applyin other kinds of machining operations).

The abruptness of the engagement angle and with the work-piece materialmay be characterised by means of a scale extending from 0 to 30, ofwhich 30 indicates the harshest interrupt condition. For example,interrupted cutting of hardened steel may be on a scale of H5 to H30,and of grey or nodular cast iron as used in brake discs for automobilesor other vehicles on a scale of K05 to K30.

The motion of the cutting tool may be characterised in terms of variousparameters. For example in so-called ‘OD’ (outer diameter) machining,the tool will be fed longitudinally along the side of the spinningwork-piece, away from one end and towards the opposite end; and in ‘faceturning’, the tool will be radially inwards as it reduces the diameterof the work-piece being cut.

The continuous engagement length refers to the length of the arc of theoutermost side of a spoke formation in units of distance or as apercentage of the circumference of the circle circumscribing the radialcross section of the work-piece. It may also be expressed in terms ofthe time over which the cutting tool engages a spoke formation. Thecontinuous engagement length may refer to the length for an individualspoke formation or the combined lengths of all of the spokes, expressedin units of distance, time or as a percentage of a full revolution.

Machining tests for tools in Interrupted machining of hardened steel(which may also be referred to as ‘hard turning’) can be divided broadlyas H05-10 (90 to 100 percent continuous cutting and lower engagementangle), H15-H20 (the tool is in contact with work-piece about 60 to 80percent of the time, per unit length in facing and turning applications,and low to intermediate engagement angle) and H25-H30 (continuousmachining is approximately 30 to 50 percent per unit length, withvarying, but approximately equidistant gaps along the face or outerside. High engagement angles close to 90 degrees are often used,depending on the configuration of the work-piece.

As used herein, a super-hard material has a Vickers hardness of at leastabout 25 gigapascals (GPa). Diamond and cubic boron nitride (cBN)material are examples of super-hard materials. A super-hard cuttersegment will comprise super-hard material, in which the cutting edgewill be at least partly defined by super-hard material. Polycrystallinecubic boron nitride (PCBN) material comprises a range of various grades(or types) of super-hard composite materials that include grains ofcubic boron nitride (cBN) dispersed within and bonded to a bindermatrix.

In some examples of PCBN material, the content of cBN grains is at leastabout 60 volume percent, at least about 70 volume percent or at leastabout 80 volume percent.

PCBN may be divided into two broad groups, namely “low cBN” and “highcBN”, in which the cBN content is about 30 to 70 volume percent andabout 70 to 95 volume percent, respectively. High CBN materials arelikely to be used for operations involving a higher degree ofinterrupted cutting, which may occur as a result of shape features ofthe work-piece or the material comprised in it. Higher cBN content tendsto result in stronger PCBN, which is especially important forinterrupted operations.

While super-hard materials are extremely hard, they are generally lessstrong and tough than cemented carbide materials, and consequently theyare more prone to fracture and chipping. Cemented carbide cutting toolsmay yield better tool life than PCD and PCBN tools due to their highertoughness and chip resistance, despite the fact that PCD and PCBN arevastly more resistant to abrasion. For example, standard texts indicatethat carbide tools with negative rake angles should be used for therough machining, or roughing, of titanium alloys when possible. Anadvantage of using PCBN tools rather than cemented carbide tools arisesfrom the superior refractory ‘hot hardness’ of the PCBN material, whichwill likely be particularly advantageous in higher speed cuttingoperations, in which the speed may be at least 150 metres per minute(m/min) and relatively higher temperatures will be generated at theinterface between the cutting tool and the work-piece.

Although cBN is relatively unreactive with ferrous metals, chemical wearof CBN grains comprised in PCBN material is likely to be evident at thehigh temperatures reached in continuous machining. Therefore, high PCBNcomprising a relatively high content of cBN grains is likely to be usedin operations such as interrupted machining, in which the tool insertmaterial needs to be relatively strong and maintain its hardness atrelatively high temperatures. PCBN material comprising a relativelylower content of cBN grains is likely to be used in operations such ascontinuous machining, in which the tool insert material needs to berelatively resistant to chemical wear. The strength of PCBN materialcomprising relatively large cBN grains is generally likely to lower thanthat of PCBN material comprising relatively small (fine) cBN grains, allelse being equal (this may be particularly evident where the content ofcBN is relatively high). Therefore, fine grain PCBN is likely to bestronger and produce a better work-piece surface finish than coarsergrain PCBN material.

In general, it may be expected that PCBN material comprising relativelycoarse cBN grains would result in too poor a surface finish of thework-piece in some applications. Therefore, the cBN comprised in PCBNmaterial for machining operations has tended not to be substantiallygreater than about 4 microns and most commercially used PCBN materialscomprise cBN grains in the range of about 1 micron to about 2 microns.Disclosed example PCBN materials (also referred to as cBN compositematerials) span a wider range of grain sizes, having mean (d50) valuesof 2 to 20 microns.

Intermediate machining operations involving a degree of interruptedcutting in combination with high machining speeds pose a challenge fordesigning PCBN material. In certain applications, for exampleapplications in which PCBN material is used to machine hardened steel inan intermediate interrupted mode (as characterised in the so-called“drilled 43/40”), there tends to be a degree of chemical as well asabrasive wear of the cBN comprised in the PCBN. The principal failuremode in such applications is chipping, which is believed to arise fromthe combination of chemical (crater) wear and impact associated with theinterrupted nature of the work-piece.

As used herein, a material that “substantially consists of” certainconstituents means that the material consists of the constituents apartfrom minor amounts of practically unavoidable impurities.

As used herein, the phrase metal silicide or metal boride, in which themetal may be specifically named, will generally refer to compoundscontaining one or more of the metal atoms and one or more of silicon orboron atom, respectively. For example, unless otherwise stated, a metalsilicide phase may comprise the corresponding silicide and ordi-silicide compound and or compounds containing three or more siliconatoms. In particular, titanium silicide will generally include TiSi andTiSi₂, and titanium boride will include TiB₂. However, specificcompounds may be mentioned, and these will generally refer tostoichiometric, sub- and super-stoichiometric forms of the compound,unless otherwise stated.

1. Composite material consisting of: at least 65 volume percent cubic boron nitride (cBN) grains dispersed in a binder matrix, the binder matrix comprising a plurality of microstructures bonded to the cBN grains and a plurality of intermediate regions between the cBN grains; the microstructures comprising nitride or boron compound of a metal; and the intermediate regions including a silicide phase containing the metal chemically bonded with silicon; in which the content of the silicide phase is 2 to 6 weight percent of the composite material; and the cBN grains have a mean size of 0.2 to 20 μm.
 2. (canceled)
 3. (canceled)
 4. Composite material as claimed in claim 1, in which the microstructures comprise titanium nitride (TiN).
 5. Composite material as claimed in claim 1, in which the microstructures comprise titanium di-boride (TiB₂).
 6. Composite material as claimed in claim 1, in which the silicide comprises a titanium silicide material having the chemical formula Ti_(x)Si_(y), where x is 0.9 to 1.1 and y is 0.9 to 1.1 (substantially TiSi).
 7. (canceled)
 8. (canceled)
 9. (canceled)
 10. Composite material as in claim 1, in which the silicide phase is spaced apart from the cBN grains by the microstructures.
 11. Composite material as claimed in claim 1, in which the microstructures are in the form of coating layers bonded to surface areas of the cBN grains.
 12. Composite material as claimed in claim 1, in which the content of the cBN grains is at least 80 volume percent of the composite material.
 13. (canceled)
 14. Composite material as claimed in claim 1, in which the cBN grains have a mean size of 0.1 to 10 microns.
 15. Composite material as claimed in claim 1, in which an area distribution of the cBN grains as viewed on a surface of the composite material has at least two modes.
 16. (canceled)
 17. (canceled)
 18. A method of using the composite material as claimed in claim 1, the method including: providing a machine tool comprising a cutting edge comprising the composite material; using the machine tool to machine a work-piece comprising ferrous material in interrupted mode, in which the work-piece is configured such that a continuous engagement length is 30 to 50 percent of a distance traversed between the cutting edge and the surface of the work-piece during the machining operation.
 19. The method as claimed in claim 18, in which the work-piece is configured such that at least a part of the work-piece presents an engagement angle to the cutting tool, the engagement angle being 30 to 90 degrees.
 20. (canceled)
 21. The method as claimed in claim 18 in which the work-piece comprises steel, cast iron or super-alloy material.
 22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled)
 27. The method as claimed in claim 18, in which the work-piece is suitable for the manufacture of a brake disc, and the method includes machining the work-piece to make a brake disc.
 28. A method of making an article comprising composite material as claimed in claim 1, the method including: combining a silicide phase precursor with a plurality of cBN grains to provide a raw material combination, the silicide phase precursor selected such that the metal will be capable of reacting with the cBN grains to form a nitride or boride reaction product; in which the content of the cBN grains in the raw material combination is such that the content of the cBN grains in the composite material will be at least 65 volume percent of the composite material; subjecting the raw material combination to a pressure at which cBN is a thermodynamically stable phase, and a temperature sufficiently high and for a sufficient period of time for some of the metal contained in the silicide phase precursor to react with the cBN grains to form the plurality of microstructures reaction-bonded to the cBN grains; and for remaining metal silicide phase to be 2 to 6 weight percent of the composite material or 20 to 60 percent of the binder matrix, being the balance of material other than the cBN comprised in the compound material.
 29. The method as claimed in claim 28, in which the silicide phase precursor is in powder form, the mean grain size of the grains of the silicide phase powder being 0.1 to 5 microns.
 30. The method as claimed in claim 28, in which the metal comprised in the silicide phase precursor is titanium (Ti).
 31. The method as claimed in claim 28, in which the silicide precursor comprises a titanium silicide material having the chemical formula Ti_(x)Si_(y), where x is 0.9 to 1.1 and y is 0.9 to 1.1 (substantially TiSi).
 32. (canceled)
 33. (canceled)
 34. The method as claimed in claim 28, including producing the silicide phase, by combining the metal and Si in elemental form such that the metal and the Si will be capable of reacting with each, forming a pre-reaction combination; treating the pre-reaction combination such that the metal reacts with the Si to form reacted material comprising the silicide phase; comminuting the reacted material to provide a plurality of the grains of the silicide phase.
 35. The method as claimed in claim 34, including comminuting the reacted material by means of attrition milling.
 36. A machine tool suitable for machining ferrous work-piece in interrupted mode, comprising composite material as claimed in claim 1, in which a cutting edge, a rake face and a flank face comprise the composite material.
 37. (canceled) 